The present invention relates to communications systems and methods, in particular, to systems and methods for recovering information from signals generated in a communications medium.
Radiotelephone systems are commonly employed to provide voice and data communications to a plurality of mobile units or subscribers. For example, analog cellular radiotelephone systems, such as designated AMPS, ETACS, NMT-450, and NMT-900, have been deployed successfully throughout the world. More recently, digital cellular radiotelephone systems such as designated IS-54B in North America and the pan-European GSM system have been introduced. These systems, and others, are described, for example, in the book titled Cellular Radio Systems by Balston, et al., published by Artech House, Norwood, Mass, 1993.
As illustrated in FIG. 1, a conventional cellular communication system 20 includes one or more mobile terminals 10 which can communicate with one or more base stations 20 serving cells 30. The base stations 20 are connected to a mobile telephone switching office (MTSO) 40, which may in turn be connected to a public switched telephone network 50. Although only two cells 20 are shown in FIG. 1, a typical cellular network may include hundreds of base stations, thousands of mobile terminals and more than one MTSO. Each cell will have allocated to it one or more dedicated control channels and one or more voice channels. Through this cellular infrastructure, a communication link may be effected between two mobile terminals 10, or between a mobile terminal 10 and a landline telephone 60, with a base station 20 typically functioning as a relay station for data and voice information.
Components of a radio communications channel connecting a mobile terminal 10 and abase station 20 which can affect communications between the terminal 10 and the base station 20 include the communications medium, i.e., the atmospheric signal path across which radio communications signals are transmitted, which may introduce fading and interference into the radio communications signals. The fading may include long-term fading due to variations in terrain along the signal propagation path, as well as short-term multipath fading due to reflections from features such as buildings which cause fluctuations in received signal strength and other distortions at a receiving station. Mobile terrestrial radiotelephone communications are particularly susceptible to short-term multipath fading because the signal pathways tend to be close to the ground. Other elements which may have an effect on communications include transmitting and receiving components commonly found in base stations and mobile terminals, such as transmitters, receivers and antennas.
Various signal estimation techniques are conventionally employed to deal with signal degradation on a channel, including diversity reception, signal coding, and specialized demodulation techniques which produce estimates of a channel transfer characteristic. Diversity reception techniques included spatial diversity reception using multiple spaced apart receiving antennas, and polarization diversity reception using multiple antennas designed to accept electromagnetic signals having particular polarizations. As signals from diverse paths typically exhibit uncorrelated fading, they may be combined in the receiver to provide a more accurate estimate of information from a received signal. Signal coding schemes typically employ redundancy to enhance the accuracy of an estimate produced from a received signal, while channel estimation techniques such as pilot tone or symbol assisted modulation and demodulation can provide improved knowledge of a transfer characteristic for the channel to aid in estimating information from a received signal.
Despite the theoretical advantages of these estimation techniques, they may provide less than optimal communication under various propagation conditions encountered in a typical cellular communications system. Diversity reception may not provide sufficient gain because of limitations on antenna size, orientation, and placement, and may be ineffective or unnecessary at particular speeds or other conditions. Coding schemes may entail an undesirable computational burden, while parameters used in estimating the channel may not be optimal for all mobile terminal speeds and under all delay spread conditions.
In light of the foregoing, it is an object of the present invention to provide communications apparatus and methods which can provide improved signal estimation.
It is another object of the present invention to provide cellular communications apparatus and methods which can provide more optimal signal estimation under differing conditions of delay spread and terminal speed.
These and other objects, features and advantages are provided according to the present invention by communications apparatus and methods in which information is recovered at one entity, e.g., a mobile terminal, from a signal generated in a communications medium by another entity, e.g., a cellular base station, by adaptively estimating the information from the signal based on a mobility characteristic which characterizes communications between the entities. The process can be bidirectional, i.e., adaptive estimation of received signal may be performed at either end of a communications channel. The mobility characteristic may characterize mobility properties of the entities which affect signal propagation between the entities, such as relative speeds of the entities with respect to one another, range between the entities, and the like. According to one embodiment, the mobility characteristic includes a cell type identifier which characterizes mobility of mobile terminals with respect to a base station, e.g., an identifier which identifies a cell served by the base station as a highway macrocell, an urban macrocell, a microcell or the like, and which bears a correlation to aggregate speed and range properties for terminals typically found within the cell. The cell type identifier may be broadcast to mobile terminals from the base station over, for example, a broadcast control channel, and the mobile terminals can adaptively estimate information from signals received from the base station based on the broadcast cell type identifier. For example, the mobile terminal may store various signal processing elements such as channel estimators, channel interpolation filters, channel trackers, and the like, as well as various parameters for such elements, and may apply these selected elements and parameters to a signal received from a base station based on a cell type identifier received from the station. Accordingly, more optimal signal estimation can be achieved by tailoring signal processing to the mobility characteristic.
In particular, according to the present invention, a communications system for communicating across a communications medium includes a first entity including means for generating a signal in the communications medium, the signal representing information, and a second entity including signal receiving means, responsive to the communications medium, for receiving a signal from the communications medium. The second also includes means for determining a mobility characteristic for communications between the first entity and the second entity, as well as adaptive estimating means, responsive to the signal receiving means, for adaptively estimating the information from the received signal based on the determined mobility characteristic.
According to one aspect of the present invention, the first entity further includes means for transmitting a mobility characteristic, and the means for determining a mobility characteristic includes means for receiving the transmitted mobility characteristic. The adaptive estimating means may include means for adaptively estimating the information based on the received mobility characteristic. The adaptive estimating means may include a number of different adaptive signal processing apparatus, including means for adaptively demodulating the received signal based on the determine mobility characteristic, means for adaptively estimating a characteristic for a channel connecting the first and second entities based on the determined mobility characteristic, means for selecting an interpolation filter based on the determined mobility characteristic, means for adaptively equalizing the received signal based on the determined mobility characteristic, means for adaptively tracking a channel connecting the base station and the mobile terminal based on the determined mobility characteristic, and means for adaptively combining a plurality of received signals based on the determined mobility characteristic.
According to an embodiment of the present invention, a cellular communications system includes a base station, a mobile terminal, and means for communicating a mobility characteristic associated with communications of the base station and the terminal, between the base station and the terminal. A first one of the base station and the mobile terminal may include means for generating a signal in a communications medium, the generated signal representing information. A second one of the base station and the mobile terminal may include signal receiving means, responsive to the communications medium, for receiving the generated signal from the communications medium, and adaptive estimating means, responsive to the signal receiving means, for adaptively estimating the information from the received signal based on the received mobility characteristic. The means for communicating a mobility characteristic may include means for broadcasting a cell type identifier from the base station, for example, over a broadcast control channel. The means for communicating a mobility characteristic may also include means for communicating the mobility characteristic on a channel specific to the mobile terminal, such as an access grant channel or a fast associated control channel.
According to another aspect of the present invention, a communications terminal includes signal receiving means, responsive to a communications medium, for receiving a signal from the communications medium. Means are provided for determining a mobility characteristic associated with the communications medium, and adaptive estimating means are responsive to the signal receiving means and the mobility characteristic determining means, for adaptively estimating information from the received signal based on the determined mobility characteristic. The means for determining a mobility characteristic may include means for receiving a mobility characteristic, and the adaptive estimating means may adaptively estimate the information based on the received mobility characteristic.
According to method aspects of the present invention, information represented by a signal generated in a communications medium by a first entity is recovered by receiving the signal from the communications medium at a second entity, determining a mobility characteristic for communications between the first entity and the second entity, and adaptively estimating the information from the received signal based on the determined mobility characteristic. The mobility characteristic may be received from the first entity at the second entity, and the information represented by the received signal may be adaptively estimated based on the received mobility characteristic. A number of signal processing functions may be adaptively performed, including adaptive demodulation, adaptive channel estimation/interpolation, adaptive equalization, adaptive channel tracking, and adaptive combining of received signals. Improved communications may thereby be achieved.